fix(mtconnect): serve an empty string-typed value as Good "" (#824)

Owner decision 2026-11-11 (Java parity): a genuinely empty observation on a
String/Reference or unauthored item is Good "" instead of BadNoCommunication.
Non-string items (numeric, bool, DateTime, arrays) keep BadNoCommunication -
the Agent supplied no value; BadTypeMismatch would wrongly blame the authored
type. The UNAVAILABLE sentinel is unchanged, and a structured observation with
no text stays BadNotSupported (structured check now precedes the empty check).
Parity matrix, quality-mapping section and drivers rule updated.

Claude-Session: https://claude.ai/code/session_01GT67AD99zQQqpLCVm3p7xk

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/MTConnectObservationIndex.cs

@@ -76,7 +76,10 @@ internal sealed class MTConnectObservationIndex
 
     private const uint Good = 0x00000000u;
 
-    /// <summary>The Agent reported <c>UNAVAILABLE</c>, or supplied no text at all.</summary>
+    /// <summary>
+    ///     The Agent reported <c>UNAVAILABLE</c>, or supplied no text at all for a non-string item
+    ///     (an empty string-typed item is a Good <c>""</c> — lmxopcua#824).
+    /// </summary>
     private const uint BadNoCommunication = 0x80310000u;
 
     /// <summary>The tag is authored but the Agent has not yet reported it.</summary>
@@ -443,10 +446,8 @@ internal sealed class MTConnectObservationIndex
         var serverTimestamp = Now;
         var sourceTimestamp = DateTime.SpecifyKind(observation.TimestampUtc, DateTimeKind.Utc);
 
-        // No text at all is the degenerate spelling of "no value": MTConnect defines UNAVAILABLE as
-        // the only way to report absence, so an empty element is not an empty string value.
-        if (string.IsNullOrWhiteSpace(observation.Value) ||
-            string.Equals(observation.Value, UnavailableSentinel, StringComparison.Ordinal))
+        // UNAVAILABLE is MTConnect's one explicit "no value" sentinel — always BadNoCommunication.
+        if (string.Equals(observation.Value, UnavailableSentinel, StringComparison.Ordinal))
         {
             return new DataValueSnapshot(null, BadNoCommunication, sourceTimestamp, serverTimestamp);
         }
@@ -456,7 +457,7 @@ internal sealed class MTConnectObservationIndex
         // Publishing that as a Good string would be noise an operator would trust. Checked before
         // the tag lookup because it is a fact about the wire shape, true whether or not the data
         // item is authored — and after the sentinel, so an unavailable data set still reports the
-        // truthful no-comms.
+        // truthful no-comms, but BEFORE the empty-text test (#824).
         if (observation.IsStructured)
         {
             return new DataValueSnapshot(null, BadNotSupported, sourceTimestamp, serverTimestamp);
@@ -464,6 +465,24 @@ internal sealed class MTConnectObservationIndex
 
         _tags.TryGetValue(observation.DataItemId, out var tag);
 
+        // A genuinely EMPTY element (no text, or whitespace the parser trimmed away) is NOT the
+        // sentinel. Owner decision on lmxopcua#824 (Java parity): on a string-typed item it is a
+        // Good empty string — an Agent can legitimately report a blank Program/Message/Block.
+        // An unauthored item coerces as String (below), so it is served the same way.
+        // On any other type (numeric, Boolean, DateTime, any array) "" cannot be a value, so it
+        // stays Bad — and stays BadNoCommunication rather than BadTypeMismatch on purpose:
+        // the Agent supplied no value at all (the same meaning as UNAVAILABLE, just spelled
+        // degenerately), whereas BadTypeMismatch tells the operator the Agent sent text of the
+        // wrong type and points them at the tag's authored data type, which is not the fault.
+        // Checked AFTER the structured test above, so element-children with no text stay
+        // BadNotSupported rather than becoming a Good "".
+        if (string.IsNullOrWhiteSpace(observation.Value))
+        {
+            return tag is null or { IsArray: false, DriverDataType: DriverDataType.String or DriverDataType.Reference }
+                ? new DataValueSnapshot(string.Empty, Good, sourceTimestamp, serverTimestamp)
+                : new DataValueSnapshot(null, BadNoCommunication, sourceTimestamp, serverTimestamp);
+        }
+
         // TIME_SERIES vectors arrive as one whitespace-separated string. Checked AFTER the
         // sentinel so an unavailable array tag still reports the truthful no-comms.
         if (tag is { IsArray: true })